Thesis (PhD) -- Stellenbosch University, 2003. / Please refer to full text to view abstract. / ENGLISH ABSTRACT: The effects of different land use practices on soil quality in South Africa were
investigated in five contrasting biomes, with a particular emphasis on the tendency of
soils to crust and soil C content. Soil quality is a nebulous concept and its
applicability in the South African landscape is scrutinised. A wide range of chemical
and physical soil properties were examined. The tendency of soils to crust was
assessed using modulus of rupture, water dispersible clay and a new method of
laboratory infiltration which was verified with rainfall simulation. Crusting was
greater in bare, exposed soils than soils under vegetation and varied with soil parent
material. Differences in crusting are explained by factors relating to clay dispersion
such as clay mineralogy, soil C, labile or readily oxidisable C, concentration of
soluble salts, soil texture and exchangeable Na percentage (ESP). Results from longterm
bum plots in savanna and grassland revealed that annual burning can increase
the tendency of soils to crust. Greater crusting in burnt plots is ascribed to greater
dispersion of clay, which in tum is attributed to a decline in soil C, a decline in EC
and an increase in ESP. The loss of nutrients from burnt plots over time is ascribed to
removal of ash in surface runoff. Calcium, Mg, and K were lost more readily than Na
probably because plants take up these nutrients in greater concentration than Na. The
net effect was an increase in ESP. Crusting on burnt plots may be self-perpetuating,
because increased runoff is likely to incsease the loss of soluble salts. Removal of
vegetation due to cultivation, grazing or burning reduced soil C at all sites. Mean soil
C in the 0-1 cm layer of unburnt plots in the Kruger National Park was more than
three times greater than in burnt plots (2.7 vs 0.8%). The difference in soil C between
treatments decreased with depth and illustrated that sampling to depths greater than a
few centimetres can obscure effects of land use. The top few centimetres of soil have
a disproportionate effect on soil infiltrability and nutrient cycling. This layer was
named the pedoderm. Tree cover on burnt plots in the southern Kruger Nationa--l- Park
is highly variable, and was hypothesised to be a function of herbivory pressure.
Herbivores tended to congregate on plots with the greatest clay, Zn and Mn content
and the lowest tree cover. It is suggested that soil properties determine the abundance
of herbivores after fire, which in tum affects tree cover. In the Eastern Cape,intensive stocking with goats transforms dense thicket to an open savanna. Soils from
goat-transformed sites had a greater tendency to crust than soils from intact thicket,
probably due to aggregate weakening associated with a decline in soil C. Mean soil C
content of intact thicket was almost double that of goat-transformed thicket (5.6 vs.
3% to a depth of 10 cm) and is exceptionally high for a semi-arid region. The
potential to sequestrate carbon in degraded thicket landscapes is thus considerable.
Managing the land for greater sequestration of C will have the added benefit of
increasing soil aggregate stability, reducing the tendency of soils to crust and
therefore increasing the rate of water infiltration through the pedoderm. The benefits
of such an approach have been recognised by specialists in soil conservation and rural
land use for many decades, based largely on empirical observation. The results of this
thesis provide a more quantitative basis for appreciating the effects of soil C across a
broad spectrum of South African biomes. / AFRIKAANSE OPSOMMING: Die invloed van verskillende landgebruikspraktyke op grondkwaliteit in Suid-Afrika
is in vyf kontrasterende biome ondersoek met spesifieke klem op die neiging van
gronde om korste te vorm en die grond koolstofinhoud. Grondkwaliteit is 'n vae
konsep en die toepassing daarvan in die Suid-Afrikaanse grondlandskap is noukeurig
ondersoek. 'n Wye reeks van chemiese en fisiese grondeienskappe is ondersoek. Die
neiging van korsvorming by gronde is beraam deur die gebruik van breukmodulus,
waterdispergeerbare klei en 'n nuwe metode van laboratorium-infiltrasie wat met
behulp van reënvalsimulasie gekontroleer is. Korsvorming was groter in kaal,
blootgestelde gronde as in gronde met 'n plantbedekking en het gewissel volgens
moedermateriaal. Verskille in korsvorming word verklaar deur faktore wat verband
hou met kleidispergering soos byvoorbeeld kleimineralogie, grondkoolstof, labiele of
maklik oksideerbare koolstof, konsentrasie oplosbare soute, grondtekstuur en
uitruilbare natriumpersentasie (UNP). Resultate van langtermyn brandpersele in
savanna en grasland het getoon dat jaarlikse brand die neiging tot korsvorming kan
verhoog. Meer korsvorming in brandpersele word toegeskryf aan groter
kleidispergering, wat waarskynlik verband hou met 'n afname in grondkoolstof, 'n
afname in elektriese geleiding (Be) en 'n toename in UNP. Plantvoedingstowwe
gaan oor tyd verlore uit brandpersele, waarskynlik deur die verwydering van as in
oppervlak afloop. Kalsium, Mg en K gaan meer geredelik verlore as Na, waarskynlik
omdat plante hierdie voedingstowwe in groter hoeveelhede opneem as Na. Die netto
effek is 'n toename in UNP. Korsvorming op brandpersele kan self instand gehou
word omdat verhoogde afloop die moontlike verlies van oplosbare soute kan verhoog.
Verwydering van plantegroei deur bewerking, beweiding of brand het grondkoolstof
op alle plekke verlaag. Die gemiddelde grondkoolstof in die 0-1 cm laag van
ongebrande persele in die Kruger Nasionale Park was meer as drie maal groter as in --..
brandpersele (2.7 vs 0.8 %). Die verskil in grondkoolstof tussen behandelings neem
af met diepte wat daarop dui dat monsterneming tot dieptes groter as 'n paar
sentimeters die effek van landgebruik kan verberg. Die boonste paar sentimeters van
'n grond het 'n oneweredige invloed op infiltrasie en voedingstofsirkulasie. Hierdie
laag word die pedoderm genoem. Boombedekking op brandperseie in die suidelikeKruger Nasionale Park is hoogs variërend. Die hipotese was dat dit 'n funksie van
druk deur planteters is. Planteters neig om op persele met die hoogste klei, Zn- en
Mn-inhoud en die laagste boom bedekking te versamel. Daar word voorgestel dat
grondeienskappe die hoeveelheid planteters na 'n brand bepaal. Dit beïnvloed op sy
beurt weer die boombedekking. In die Oos-Kaap het intensiewe bokboerdery digte
bosruigtes verander na oop savannas. Gronde van bok-veranderde lokaliteite het 'n
groter neiging tot korsvorming as gronde van onveranderde bosruigtes, vermoedelik
as gevolg van verswakking van aggregate met 'n. afname in grondkoolstof. Die
gemiddelde grondkoolstof van onveranderde bosruigtes was byna dubbel soveel as die
koolstof van bok-veranderde bosruigtes (5.6 vs 3 % tot 'n diepte van 10 cm) en
buitengewoon hoog vir 'n semi-ariede streek. Die potensiaal vir koolstof sekwestrasie
in degradeerde bosruigte landskappe is dus aansienlik. Bestuur van land vir groter
sekwestrasie van koolstof het die bykomende voordeel van verhoogde grond
aggregaatstabilteit, verlaging van die neiging tot korsvorming en daardeur 'n
verhoging in die tempo van waterinfiltrasie deur die pedoderm. Die voordele van so
'n benadering is vir baie dekades deur spesialiste in grondbewaring en landelike
landgebruik herken. Dit was grootliks gebaseer op empiriese waarneming. Die
resultate van hierdie tesis bied 'n meer kwantitatiewe basis tot die verstaan van die
invloed van grondkoolstof oor 'n breë spektrum van Suid-Afrikaanse biorne.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/53567 |
| Date | 12 1900 |
| Creators | Mills, A. J. |
| Contributors | Fey, M. V., Stellenbosch University. Faculty of AgriSciences. Dept. of Soil Science. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | Unknown |
| Type | Thesis |
| Format | 152 leaves : ill. |
| Rights | Stellenbosch University |
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